configure.ac: Detect wchar_t using wchar.h header
configure.ac: Detect wchar_t using wchar.h header

The wchar_t type is defined in `wchar.h` and if this header is not
included by autoconf the detection of the type will fail.  This type is
required by `unicharset_extractor` to autogenerate the character
properties.

This problem was detected when running under Fedora 21.

UB-Mannheim/tesseract,UB-Mannheim/tesseract,tesseract-ocr/tesseract,stweil/tesseract,UB-Mannheim/tesseract,stweil/tesseract,stweil/tesseract,jbarlow83/tesseract,amitdo/tesseract,UB-Mannheim/tesseract,stweil/tesseract,jbarlow83/tesseract,UB-Mannheim/tesseract,stweil/tesseract,tesseract-ocr/tesseract,amitdo/tesseract,jbarlow83/tesseract,amitdo/tesseract,amitdo/tesseract,jbarlow83/tesseract,tesseract-ocr/tesseract,tesseract-ocr/tesseract,jbarlow83/tesseract,amitdo/tesseract,tesseract-ocr/tesseract